Moisture-curable compositions based on silane-functional polymers are widely used as mainly elastic adhesives, sealants, or coatings. Compared to curable polyurethane compositions which in many cases are used for the same applications, such compositions based on silane-functional polymers are generally less problematic concerning health and safety issues. Sealants, adhesives, or coatings based on silane-functional polymers can be formulated both in one- or two-component compositions. One-component compositions are conveniently stored in single packing units and their curing mechanism is triggered as soon as humidity, mostly from air, gets in contact with the composition during and after application. This however requires careful formulation of the compositions, including strict drying of all components and addition of significant amounts of drying agents, in order to obtain sufficient shelf life of the products by preventing preliminary curing in the package. This often increases the production costs considerably and requires an advanced process control during production. Furthermore, if a long pot-life during application is required, such one-component compositions have to be deactivated considerably and require excessive time for proper through cure.
Two-component compositions based on silane-functional polymers try to get around these problems by providing a second component which often contains the curing catalyst. The two components are mixed immediately before application, and the water which is mixed into the composition is directly used to cure the composition in a very homogeneous way. Compared to single component compositions, the advantage of two-component compositions is typically a longer open time in combination with faster through cure, as curing does not only rely on moisture from ambient air slowly diffusing in. These two properties are easy to combine in a two-component composition which further maintains a long shelf life because the reactive components, i.e. catalyst and silane-functional polymer, are stored separately.
Depending on the application, the formulation can be adapted in order to achieve the desired final properties of the cured product. For elastic joint sealants for example, good adhesion and low E-modulus is desirable. Such a low modulus sealant is able to compensate for strong movement of the joint, e.g. due to thermal expansion, without breaking, peeling off or damaging the substrate.
Recent developments have led to two-component compositions based on silane-functional polymers with fast through cure and promising mechanical properties suitable as joint sealants. However, especially if they are formulated with highly reactive trifunctional silane polymers, such as trimethoxysilane-terminated polymers, the component containing the highly reactive polymers requires careful drying of ingredients, such as fillers, in order to obtain an acceptable shelf life. Such sealants in general often suffer from poor adhesion properties and require the use of adhesion promoters if used without pretreatment. On the other hand, compositions primarily based on less reactive bifunctional silane polymers, such as methyldimethoxysilane-terminated polymers, do generally offer better shelf life, but they are slower in curing and are often not able to fulfill the most demanding mechanical requirements in terms of resilience, tensile strength, and durability. Therefore there is still a demand for a simple, inexpensive two-component solution for joint sealants based on silane-functional polymers, which combines the advantages of low E-modulus, fast curing, long shelf life, excellent adhesion properties and outstanding durability in high movement joints, without the need for adhesion promoters, drying of the components, or expensive packaging.